A wide variety of RDPs, which may be made by spray drying an emulsion polymer in the presence of various additives such as a protective colloid and anti-caking agent, are well known in the art and are available from commercial sources. Examples of conventional RDPs, together with processes for manufacturing such compositions, include those disclosed in U.S. Pat. Nos. 5,403,894, 5,739,179, 6,559,236 and 6,624,243.
Multi-layered exterior wall systems such as exterior insulation and finish systems (EIFS) are used extensively as exterior wall surfaces in commercial and residential buildings. Such multi-layered systems are conventionally prepared by covering a substrate such as plywood or gypsum board with a layer of insulation material, covering the insulation material with a cementitious basecoat which is embedded with a reinforcing mesh and finishing with a non-cementitious topcoat or exterior finishing composition which is typically applied with a trowel to form a layer with a thickness of from 0.1 to 2.5 cm. Such exterior finishing compositions not only provide insulation but also act as a barrier to moisture ingress and provide a decorative function. Therefore, to be suitable for exterior application, such topcoat or finishing compositions need to have good exterior durability, including good colour retention, dirt pickup resistance, water resistance and abrasion resistance.
Conventional exterior finishing compositions or textured paints typically contain a film-forming polymer to bind the pigments and extenders together into a tough, durable coating. Consequently, such finishing compositions are typically supplied as a wet mix because the polymer itself is supplied in an aqueous form. However, in order to simplify transportation and eliminate freeze-thaw issues associated with liquid materials, the polymer may be transported in dry form and combined with other dry components of the finishing composition to form a dry mix formulation to which water may be added at the point of use.
The use of RDP binder compositions in external finishing compositions is disclosed, for example, in EP-A-0721004 which provides a RDP composition for use in water-resistant films and coatings. The RDP comprises a polymer containing carboxyl, hydroxyl, carboxylamide or amine functionality and a reactive component that forms a non-ionic bond with said polymer functional group following dispersion of the admixture in water. Further examples of RDP-containing external finishing compositions can be found in WO-A-2012/007529 and U.S. Pat. No. 6,228,937.
However, although the provision of such dry mix formulations is advantageous with regard to transportation, it has been found that textured paints prepared in this way using commercially available acrylic RDPs, e.g. DRYCRYL™ DP-2903 (a 100% acrylic RDP having a Tg of +10° C. commercially available from the Dow Chemical Company, USA), provide reduced performance, specifically dirt pick up resistance and/or wet abrasion resistance, compared with textured paints prepared from liquid polymers, e.g. RHOPLEX™ EI-2000 (a 100% acrylic polymer latex having a Tg of +12° C., commercially available from the Dow Chemical Company, USA), which is specifically designed for use in exterior finishing compositions and provides colourfastness and dirt pick up resistance in EIFS topcoat formulations.
It is therefore an object of the present invention to provide a water-redispersible polymer powder (RDP) that, when incorporated into a external finishing composition such as a topcoat formulation of an external insulation finishing system (EIFS), provides improved dirt pickup resistance without a loss in wet abrasion resistance (which is considered to be an early indicator of exterior durability) over conventional textured paint formulations comprising acrylic latex powders and, preferably, acrylic latexes. Preferably, the external finishing composition provides an improvement in both dirt pickup resistance and wet abrasion resistance over conventional textured paint formulations comprising acrylic latex powders and, even more preferably, over those comprising acrylic latexes.